VENICE (AFP) - President Barack Obama said Sunday that British energy giant BP was clearly "responsible" for the giant oil spill that threatens US shores and must therefore pay for the cleanup.

"So let me be clear," Obama told journalists during a visit to the stricken Louisiana coast. "BP is responsible for this leak. BP will be paying the bill."

"But as president of the United States, I'm going to spare no effort to respond to this crisis for as long as it continues," he added.

Barack Obama on Sunday called a widening oil slick "potentially unprecedented" for the environmental and economic damage it may inflict on the US Gulf's vulnerable shores.

"Now, I think the American people are now aware, certainly the folks down in the Gulf are aware, that we're dealing with a massive and potentially unprecedented environmental disaster," Obama said.

"The oil that is still leaking from the well could seriously damage the economy and the environment of our Gulf states," he told journalists during a visit to the stricken Louisiana coast on the Gulf of Mexico.

"And it could extend for a long time. It could jeopardize the livelihoods of thousands of Americans who call this place home," the president added.